Narrated Abu Musa:
A man embraced Islam and then reverted back to Judaism. Mu'adh bin Jabal came and saw the man with Abu Musa. Mu'adh asked, "What is wrong with this (man)?" Abu Musa replied, "He embraced Islam and then reverted back to Judaism." Mu'adh said, "I will not sit down unless you kill him (as it is) the verdict of Allah and His Apostle(Sahih Bukhari Vol.09,Book 89,No.271